You will find it difficult to get a visa unless you can prove your in a relationship AND she as good reason to return to Thailand, such as she owns a house, has money in the bank, has a job where the employer gives a letter saying she will return and any other evidence she will return.
Its a prescription drug in most countries and therefore not normally available over the counter (but worth trying) a local doctor, private hospital should be able to help, just book an appointment with the doctor, but private hospitals are more expensive
The UK are looking evidence that the person will return back home: such as: they own a property, have a job and letter saying they will return, a local bank account with money in it, without this they will not be successful.